Fibre-based packaging materials for containers and product packages, such as packaging paper or board, are usually provided with a polymeric coating that makes the material liquid-tight and allows forming the container or package by heat sealing. Typical articles made from polymer coated paper or board are liquid packages and disposable tableware, such as drinking cups. LDPE (low-density polyethylene) is widely used for coating due to its good heat-sealability.
Recently there has been a growing demand on the market for such paper or board products made solely or at least predominantly of raw-materials from renewable sources, i.e. raw-materials of biologic origin. Traditionally polyethylenes have been produced from fossile raw materials such as petroleum. More recently polyethylenes made from sugarcane, sugar beet or wheat grain have been developed, especially bio-LLDPE (linear low-density polyethylene) and bio-HDPE (high-density polyethylene) having easy availability. Also bio-LDPE, i.e. usual low-density polyethylene with a branched structure, is known but has limited supply and accordingly a high price. All these new polymer products have so far been tailored for the manufacture of polymer films, and their suitability to extrusion coating has been found to be poor.
HDPE in general has an advantage over LDPE or LLDPE in providing a superior water vapour barrier, which is very desirable in liquid containers and packages. On the other hand HDPE has a poor heat-sealability due to its higher melting temperature as compared to LDPE, and even its adhesivity in direct contact to a fibrous base is deficient. Furthermore, pure HDPE is not suitable for monolayer extrusion coating due to its narrow molecular weight distribution (MWD). Pure HDPE has a high neck-in and poor runnability in extrusion coating, when producing thin coatings of a coating weight of 15 to 25 g/m2.
Multilayer coatings co-extruded onto a fibrous base are widely applied for fulfilling multiple goals such as water vapour barrier, oxygen and aroma barrier, adhesivity, heat-sealability etc. In the prior art, see e.g. U.S. Pat. No. 7,335,409, a combination of an inner HDPE layer and an outer LDPE layer has been described as providing a water vapour barrier and heat-sealability. However, adhesivity of such combination to a fibrous base is insufficient, and the present poor availability of bio-grade LDPE is another handicap from the environmental point of view.